One embodiment relates to a method for the manufacture of synthetic quartz glass. One embodiment also relates to a polyalkylsiloxane compound, which includes certain specifications with respect to chlorine content, metallic impurities content, and residual moisture, as well as the use thereof for the production of synthetic quartz glass. A further aspect of one embodiment relates to a synthetic quartz glass that can be obtained according to the method according to one embodiment.
Referring to the production of synthetic quartz glass, it is customary to generate SiO2 particles from a silicon-containing starting substance in a CVD procedure by means of hydrolysis or oxidation and to deposit these on a moving support. The method can be subdivided into external and internal deposition methods. In external deposition methods, the SiO2 particles are applied onto the outside of a rotating support. Examples of pertinent external deposition methods include the so-called OBD method (Outside Vapor Phase Deposition), the VAD method (Vapor Phase Axial Deposition) or the PECVD method (Plasma Enhanced Chemical Vapor Deposition). The best-known example of an internal deposition method is the MCVD method (Modified Chemical Vapor Deposition), in which SiO2 particles are deposited on the internal wall of a tube heated from outside.
If the temperature in the area of the support surface is sufficiently high, the SiO2 particles vitrify directly, which is also known as “direct vitrification”. In contrast, the temperature during the deposition of the SiO2 particles in the so-called “soot method” is so low that a porous SiO2 soot layer is obtained, which is then sintered into transparent quartz glass in a separate process step. Both the direct vitrification and the soot method lead to a dense, transparent, highly pure, synthetic quartz glass.
Silicon tetrachloride (SiCl4), for example, is known from the prior art as a silicon-containing production material for the production of synthetic quartz glass. Silicon tetrachloride and other analogous chlorine-containing substances possess sufficient vapor pressures already at moderate temperatures below 100° C. such that any impurities usually remain in the liquid phase and the production of highly pure soot bodies is made easier.
One significant disadvantage of the chlorine-containing production materials is that the conversion thereof to synthetic quartz glass is associated with the production of hydrochloric acid, which causes high costs in the scrubbing of exhaust gas and the disposal. In the past, a large number of so-called chlorine-free organo-silicon compounds have been tried as starting materials for quartz glass production for this reason. Examples include monosilanes, alkoxysilanes, siloxanes, and silazanes. A particularly interesting group of the so-called chlorine-free organo-silicon compounds, the polyalkylsiloxanes (also called “siloxanes” for short), are known, for example, from DE 30 16 010 A1. Specifically the polycyclosiloxanes, which can be included in the polyalkylsiloxanes, are characterized by having a particularly high fraction of silicon per weight fraction, which contributes to the economic efficiency of their use in the production of synthetic quartz glass. Due to the availability of commercial quantities at high purity, octamethylcyclotetrasiloxane (OMCTS), for example, is used broadly.
The polyalkylsiloxane compounds can be polymerized and are typically present in the production material as the pure substance or, typically in liquid form, as a mixture including other compounds They can be supplied in liquid form to the consuming unit, for example to a deposition burner. But usually, the liquid production material is transferred to a gaseous or vaporous phase by means of an evaporator and is supplied as a continuous gas flow to the consuming unit through a conduit system.
Based on these so-called chlorine-free starting materials, the prior art describes a number of methods for the production of synthetic quartz glass. Reference shall thus be made, for example, to the specifications, EP 0 760 373 A, WO 99/15468 A, WO 99/54259 A, WO 2013/092553 A, and EP 0 529 189 A.
Although prior methods recommend the use of purified starting materials, they are, in part, associated with a disadvantage in that the production is associated with the formation of a precipitate or gel in the evaporator and the vapor-conducting conduit system.
It is evident from the overall assessment of the prior art that the chlorine content, the metallic impurities content, and the residual moisture in the starting material, each individually, are of significance in the production of synthetic quartz glass in order to prevent the formation of a gel. However, none of the assessed documents illustrates that there might be a functional relationship between the individual impurities with regard to the formation of a gel. Moreover, the metallic impurities that are actually responsible for gel formation are not specified.
The issue of gel formation during the production of synthetic quartz glass based on polyalkylsiloxane is evident, for example, in processes on a large technical scale, for example, in processes with a production quantity exceeding 1 t, and leads to the vapor-conducting pipelines, mass flow regulators, chokes, and corresponding valves being coated and, in part, clogged by gels and residues. This impairs the control and flow behavior of the vapor-conducting overall system in uncontrollable manner. Accordingly, increasing gel formation makes it more difficult to control the flow of the polyalkylsiloxane vapor, which in turn has a negative impact on the reproducibility of the method (for example, batch variation of the soot density distribution) and the resulting product properties (for example, uniformity of axial/radial homogeneity of the soot density), for example, in multi-burner set-ups. Moreover, high costs arise due to the need for cleaning of the facilities used in the process, possible need for replacement thereof, and corresponding downtimes during cleaning. Increased costs due to losses of material also need to be taken into account.
An additional issue is evident in soot deposition methods, in which multiple consuming units need to be supplied with the production material concurrently, such as, for example, in soot deposition by means of multi-burner set-ups. In order to prevent non-uniform soot deposition and the formation of layers, it is particularly important in this context that each deposition burner has the same soot structure characteristics, both quantitatively and qualitatively. Fluctuations of the gas supply, for example due to depositions being formed due to gel formation, should be avoided.
The documents of the prior art assessed above and the methods described in these documents cannot fully solve these problems and there continues to be a need for further improved methods for the production of synthetic quartz glass.